When melt-welding a refractory metal having a melting point of at least 2500° C., such as tungsten (W), molybdenum (Mo), tantalum (Ta), rhenium (Re), and alloy consisting primarily of these metals, or a refractory metal having a melting point of at least 2000° C., such as iridium (Ir) and alloy consisting primarily of this metal, the welding part forms a solidification structure and becomes polycrystalline with coarse grains. The refractory metallic materials described above have brittle crystal grain boundaries, which is a cause of a fatal defect in the welding part. Therefore, welding the above refractory metallic materials is conventionally performed mainly by means of rivet welding.
On the other hand, a technology for welding a metallic material by means of friction stir welding (FSW) has been known as a method for welding a metallic material. When performing the friction stir welding, metallic materials to be boned are placed to face a welding part. Then, a probe provided at a leading end of a rotary tool is inserted into the welding part, and the rotary tool is rotated and moved along a longitudinal direction of the welding part and the metallic materials are subjected to plastic flow by frictional heat to weld the two metallic materials to each other (see Patent Literature 1, for example).
Patent Literature 1: Japanese Patent Application No. 2792233
Although friction stir welding can basically achieve good welding strength, friction stir welding is generally employed for low-melting metals such as aluminum (Al) and magnesium (Mg). Hence, application of friction stir welding is limited to ferrous materials in the research steps, and friction stir welding is not performed on the above refractory metals.